Interactions of potassium and magnesium
Balanced fertilization promotes healthy plant growth. For a targeted application of fertilizer, it is important to consider not only the nutrient requirements of the crop in question, but also the nutrient conditions in the soil. It is important to consider interactions between nutrients and beneficial elements (such as sodium) so that the nutrients provided by fertilization can also be taken up by the plants.
What are the interactions of plant nutrients?
Nutrient interactions can have positive and negative effects on nutrient uptake. As the figure (PDF download of the graphic) shows, a diverse range of effects between nutrients and beneficial elements is created as soon as they come into contact with each other in soil or in plants. The arrowhead shows the direction and strength of the interaction. Sometimes there are also mutual interactions, for example, between potassium and sodium. Strong interactions are marked with thicker, solid arrows (red here).
Some nutrients favor the availability of another nutrient present in soil, promote its uptake by plants or its function in plant metabolism. A positive interaction - also called synergism - arises. In other cases, a particular nutrient inhibits the availability, absorption or function of another nutrient. This is known as negative interaction or nutrient antagonism.
The antagonism between potassium and magnesium explained in the video is particularly relevant in practice. As this can reduce harvest yield and quality, special attention must be paid to balanced fertilization for both macronutrients.
How does potassium influence the absorption of magnesium?
Potassium (K) has an antagonistic, i.e. inhibiting, effect on the absorption of magnesium (Mg). This relationship is particularly interesting because an excess supply of potassium inhibits the absorption of magnesium at the plant root, but conversely, high magnesium levels in the soil solution do not impair potassium absorption. Therefore, it is a unilateral antagonism.
How do plants absorb potassium and magnesium?
Nutrients are absorbed through the soil into roots and plants via what are called transporters. There are fundamentally different absorption mechanisms for potassium and magnesium. Magnesium ions only enter plants via a single route - via what are termed non-specific transporters. Non-specific means that the route can also be used by other nutrients, such as potassium. Potassium, on the other hand, uses various routes. In addition to the non-specific transporters, it also reaches the roots via specific transporters, which only absorb potassium into the plant. This is the reason why a very large supply of potassium can interfere with the magnesium uptake, but a high degree of magnesium availability in the soil has no influence on the potassium uptake of the plant.
How can antagonism be avoided in practice?
In practice, potassium fertilization should also take into account soil magnesium content. Depending on the result of the soil sample, it is recommended, in many cases, to use potassium in combination with magnesium or to balance a high potassium supply from organic substances by means of targeted magnesium fertilization. In this way, potassium-magnesium antagonism can be avoided.
If, on the other hand, larger amounts of potassium are used without the simultaneous application of magnesium, this procedure can shift the nutrient ratio of the root surface to the disadvantage of magnesium: The plant suffers from magnesium deficiency, although sufficient magnesium is available in the soil. The reason for this is that potassium is also present, which inhibits the absorption of magnesium. There is no such converse interaction.
What is the optimum soil K-Mg ratio?
Experience from extensive field trials has shown that with a soil potassium/magnesium ratio of 3:1, the absorption of both nutrients can be guaranteed. This is also ensured by the respective concentration ranges in soil content class C.
In deviation from this, the ratio should be somewhat closer (2.5:1) in viniculture to prevent the risk of stalk paralysis. In fruit and vegetable growing, the range of the optimal nutrient ratio ranges from 2 - 5:1 depending on the crop. These nutrient conditions in the soil ensure a high yield and that the harvested products are of good quality.
Always combine potassium and magnesium
A one-sided oversupply of potassium inhibits the uptake of magnesium, resulting in antagonism. Conversely, a high supply of magnesium has no negative effect on potassium uptake, since plants can always meet their potassium requirements via specific transporters. For the ideal uptake of both nutrients in practice, a potassium-magnesium ratio of 3 : 1 should be aimed for in the soil. With a combined potassium and magnesium fertilization you play it safe.